The British drugmaker said on Thursday it had filed a declaratory judgment action at the U.S. District Court for the Southern District of Florida arguing the "Cabilly" patent was invalid,

unenforceable and not infringed by Arzerra.

Genmab, a Danish biotech company whose prospects hinge on the success of Arzerra, said the legal case was not expected to affect its financial guidelines for 2009.

The patent has been in dispute before.

In February, the U.S. Patent and Trademark Office confirmed that Genentech's patent was valid, in a reversal of an earlier decision.

A year earlier, the patent office had rejected the validity of the Cabilly patent, prompting Genentech to appeal. The patent remained in effect during that reexamination process. (Reporting by Ben Hirschler; Editing by Hans Peters)